1. Field of the Invention
The present invention relates to a drive control system for a hybrid vehicle which is equipped as its prime mover with an internal combustion engine such as a gasoline engine or a Diesel engine and an electric motor such as a motor or a motor/generator to be energized by an electric power to output a torque. More particularly, the invention relates to a drive control system for the hybrid vehicle of the type capable of employing both the internal combustion engine and the electric motor as the prime mover for its run.
2. Related Art
The hybrid vehicle is a vehicle which has been developed for improving the deterioration of exhaust gases or the reduction in the fuel economy when it is run with only the internal combustion engine. On the hybrid vehicle, there is mounted such a motor/generator together with the internal combustion engine as acts as a motor or a generator for generating the running torque with the electric power. As the vehicle of this kind, there are known both the so-called "series" hybrid vehicle employing the internal combustion engine exclusively as the prime mover for generating the electric power and the so-called "parallel" hybrid vehicle capable of employing the internal combustion engine as the prime mover for the run, too.
On the other hand, the internal combustion engine cannot be started merely by feeding it with a fuel but has to be forcibly turned by an external force before the start. For this, the internal combustion engine is generally equipped with a motor called the "starter". In the vehicle of the parallel hybrid type, the internal combustion engine can be coupled together with the electric motor to a power transmission line for the run, so that it can be started by turning it forcibly with the electric motor. In order that the exhaust gases may not be deteriorated at a low speed such as at the start while improving the fuel economy, therefore, the vehicle is run with the electric motor. When the internal combustion engine is to be started at a considerably high running speed, moreover, the output of the electric motor being used for the run is transmitted to the internal combustion engine. As a result, the internal combustion engine can be started without employing the starter of the prior art. In other words, the starter can be abolished to reduce the number of parts.
A system for effecting the start control of the internal combustion engine is disclosed in Japanese Patent Laid-Open No. 9-193676. In the disclosed system, there is provided a drive system which is constructed by coupling the internal combustion engine through an input clutch to a predetermined rotary element in a planetary gear mechanism, by coupling the motor/generator to another rotary element, and by using a third rotary element as an output member. In the system of the prior art, moreover, while the vehicle is being run with the output of the motor/generator, the input clutch is applied to transmit the torque to the internal combustion engine so that the internal combustion engine is forcibly turned and started.
According to the system of the prior art described above, the internal combustion engine can be started without using the starter. When the input clutch is applied to rotate the internal combustion engine, however, the output torque of the electric motor, as used for the run, is consumed partially as a torque (as will be called the "motoring torque" and "inertia torque") for rotating the internal combustion engine so that the driving force temporarily drops. In other words, the running torque drops according to the application of the input clutch, and this drop may be physically felt as a shock.
After the internal combustion engine was started, on the other hand, the vehicle is run by both the electric motor and the internal combustion engine. It is, therefore, an ordinary practice to apply the input clutch completely after the revolution speeds of the electric motor and the internal combustion engine became equal. Even if the revolution speeds of the electric motor and the internal combustion engine are equal, their changing rates (or their rising rates) may be different from each other. Then, even after the input clutch was completely applied, one prime mover having a lower rising rate of the revolution speed, i.e., the electric motor or the internal combustion engine may be dragged by the other prime mover having a higher rising rate of the revolution speed. As a result, the complete application of the input clutch may cause a situation as if the running resistance were raised, and the shock may arise due to the drop of the driving force.
While the vehicle is being run by the electric motor, moreover, the input clutch is applied to raise the revolution speed of the internal combustion engine, and the fuel is fed to start the internal combustion engine. In this case, the output torque, as generated by the combustion in the internal combustion engine, is added to the torque for the run. If the input clutch then has a sufficient transmission torque capacity, therefore, the addition of the output torque of the internal combustion engine to the running torque of the electric motor will raise the driving force abruptly, and this rise may be physically felt as the shock.
Here, the input clutch can be exemplified by a multi-disc friction clutch to be hydraulically applied, as in the foregoing Japanese Patent Laid-Open. When the clutch of this kind is to be applied, valves in an oil pressure circuit are switched to feed the oil pressure from its source to the clutch. In this case, a line resistance unavoidably arises to cause a time delay till the input clutch is actually applied in response to an instruction for the application. In the input clutch, moreover, there is a clearance between the friction discs and between the friction discs and a piston for pushing the former. When the input clutch is fed with the oil pressure, therefore, the torque is not transmitted between the friction discs before the clearance (or pack clearance) is clogged. During this clogging of the pack clearance, the internal combustion engine cannot be rotated to cause the time delay in the control for rotating the internal combustion engine. Because of this delay factor in the mechanical construction, the responsibility of the control for starting the internal combustion engine may be degraded.
Even after the revolution speed of the internal combustion engine was raised by feeding it with the fuel to such a value as to continue the combustion, the internal combustion engine neither generates the torque instantly at the start of the fuel feed nor effects the continuous run, but the generation of the torque may be delayed by the influences such as the temperature of the internal combustion engine or the ambient temperature. This delay may deteriorate, together with the delay in the action of the input clutch, the responsibility of the start control of the internal combustion engine, or the rise in the driving force may be delayed to cause the so-called "slow" feeling.